Low-damage solutions

Exciting new technologies will not only improve the seismic performance of steel construction, they will enable structural elements damaged in a severe earthquake to be easily and affordably replaced.

The Canterbury earthquakes highlighted the importance of seismically resilient building construction. Steel-framed buildings, on the whole, bore the earthquakes very well, even though the shaking was significantly greater than the design level – they not only satisfied their mandate to protect lives, but were also back in service shortly after the earthquakes. The HSBC Tower is an exemplar of this.

Engineers learned a great deal from the performance of these structural steel buildings, and continue to improve their designs to ensure new buildings are not only safer for their occupants, but also avoid severe and expensive damage.

Low-damage seismic-resisting technology

The earthquakes have brought about a paradigm shift in building design philosophy. This is driving the development and uptake of new low-damage seismic-resisting technologies that can withstand significant earthquakes and require little or no major post-earthquake structural repair.

Notably, low-damage seismic-resisting technology does not come at a significant cost premium. In a recent project, the additional cost of applying low-damage systems in lieu of a conventional approach was just over 0.5% of the total building cost.

In New Zealand steel buildings, the low-damage solutions utilised to date have included the following:

Braced frames with controlled rocking

These systems employ rocking and energy dissipaters to resist severe shaking in an earthquake. In New Zealand, the award-winning Te Puni Village project at Victoria University has a braced frame with controlled rocking using ringfeder springs on concentrically braced frames and sliding hinge joints on moment-resisting frames. Since then, a 16-storey apartment in Wellington has been built using this system, and a medical centre in Christchurch has been constructed employing a post-tensioned rocking braced frame solution as the seismic load-resisting system.

Eccentrically braced frames (EBFs) with removable links

Seismic energy is dissipated by yielding of the active link zone between the intersection of the braces and the connector beam. The removable link features a bolted moment endplate connection to allow easy on-site removal and replacement after a major earthquake, if required. This technology has been well-researched internationally. At the University of Toronto, for example, the link was subjected to full-scale testing, where it successfully demonstrated satisfactory levels of ductility and an ability to safely contain the damage.

In a first for New Zealand, two new office buildings at 335 Lincoln Road in Christchurch feature bolted eccentrically braced frame (EBF) links that, if damaged in an earthquake, can be easily and cost-effectively replaced – much like changing a fuse in a circuit box.

For more information about the design of EBFs with removable links, see SCNZ’s Steel Advisor articles.

Asymmetric friction connections

The innovative asymmetric friction connection (AFC) is a fully tensioned, slotted and bolted connection that relies on frictional force between its components to provide joint strength. The AFC provides a rigid connection until the design-level earthquake is exceeded at which point the joint slides, dissipating seismic energy as friction between the sliding surfaces. The only likely structural repair is to replace stretched bolts. To date, the AFC joint has been in moment-resisting frames but is being considered for several building projects in concentrically braced frame applications.

Buckling restrained braces

This system has been used internationally for two decades and is now probably the most popular form of lateral load resisting system in New Zealand. The buckling restrained brace (BRB) behaves consistently in both compression and tension. It is manufactured with two main components that perform distinct functions while remaining decoupled: the load-resisting element is a steel core that is restrained against buckling by an outer casing filled with grout. In the event they are damaged in a severe earthquake they can be easily removed and replaced.

Following the Canterbury earthquakes there has been greater interest in this system with BRBs specified for projects at the University of Auckland, where the technology has also been the subject of research.

Viscous dampers

Viscous braced dampers were originally developed as shock absorbers for the defence and aerospace industries but have now been used extensively internationally for new and retrofit building construction in seismically active regions.

During a severe earthquake the devices are activated and seismic energy is converted to heat and dissipated. The principle benefit of introducing viscous dampers to a steel-framed building is that floor displacements and accelerations are reduced. Other low-damage solutions do not typically reduce floor accelerations – something which is important for minimising content damage, particularly to sensitive equipment. This technology is being used on a current Christchurch office building project.

Linked column frame

Another system meeting the objectives of performance based seismic design; reduced structural damage and down time in addition to collapse prevention, is the linked column frame. To date this system has not been implemented in New Zealand or internationally.

The linked column frame is a twist on old technology. A hybrid of eccentrically braced frames (EBFs) and moment-resisting frames, it features linked columns with removable energy-dissipating active links and elastic moment frames that work to bring the building back to plumb after an earthquake. This solution provides designers with a brace-free alternative to EBFs.

The eccentrically braced frame with removable link features
a bolted moment endplate connection to allow easy on-site
removal and replacement after a major earthquake, if required

Image courtesy of Ruamoko Solutions.

The linked column frame is a hybrid of eccentrically braced
frames and moment-resisting frames

Image courtesy of Peter Dusicka.

The Victoria University Te Puni Village project featured a
world-first rocking steel frame, which used pre-stressed
Ringfeder springs

Image courtesy of Aurecon.

Braced frames with sliding hinge joint: these have significant
post-elastic stiffness, which encourages re-centering of the
structure after an earthquake